1. Field
This disclosure generally relates to use of telecommunications signals to determine the location of a remote device. More specifically, the disclosure pertains to systems and methods for determining the location of a mobile communication device using multiple pilot signals.
2. Background
Today, telecommunication signals are commonly transmitted using cellular systems. Cellular systems comprise groups of cellular base stations, each of which is used to transmit signals to and receive signals from a mobile device, such as a cellular telephone, laptop computer or other such mobile device. In addition to transmitting a variety of voice and/or data signals between the mobile device and the base station, cellular system are often used to locate such mobile devices, both for emergencies and non-emergencies. For example, in the case of a call to the emergency number 911, it is frequently helpful to determine the location of the caller so that assistance can be dispatched to the caller immediately and without requiring that the caller know his/her location. In non-emergency cases, it is frequently desirable to determine the location of a mobile device to provide services such as roadside assistance, turn-by-turn driving directions, concierge services, location-specific billing rates and location-specific advertising, among others.
The CDMA protocol operates using a variety of channels. A Forward CDMA channel carries user and signaling traffic, a pilot signal, and overhead information, from a base station to a mobile device. The pilot and overhead signals establish the system timing and station identity. The pilot channel also provides a signal strength reference that is used in the mobile-assisted handoff (MAHO) process. All base stations have the same pilot waveform and are distinguished from one another only by the phase of the pilot signal.
In current systems, pilot phases are assigned to stations in multiples of 64 chips, giving a total of 512 possible assignments. A number referred to as the Pilot Offset identifies the pilot phase assignment. Different base stations are identified by different pilot pseudonoise (PN) sequence offsets.
Various techniques have been used to determine the location of a mobile device. For example, the Global Positioning System (GPS) is a satellite system that provides users equipped with a GPS receiver the ability to determine their location anywhere in the world. A GPS receiver normally determines its location by measuring the relative times of arrival of signals transmitted simultaneously from multiple GPS satellites.
Another well-known position location technique is Advanced Forward Link Trilateration (AFLT). The AFLT technique is based on measuring time-of-arrival differences between terrestrial base station pilot signals. In the case of a CDMA wireless network, these measurements are called pilot phase measurements. Whenever the mobile device is able to detect signals from a sufficient number of different base station locations, one of which is likely to be the serving base station, the mobile device's position can be determined with a certain accuracy.
It is possible that at a particular location, the mobile device is neither able to detect signals from at least four GPS satellites, or to detect signals from a sufficient number of positioning satellites or base stations. Position determination capability is compromised at these locations.